Humic substances (HS) play an important role in the reactivity and transport of colloids in natural environments. In particular, the presence of fulvic acids (FA) in natural waters modifies the interactions between inorg. particles and biopolymers and makes difficult to predict their stability with regard to aggregation processes. Brownian dynamics (BD) modeling is applied to quantify the interactions between neg. charged FA and (1) a pos. charged inorg. particle and (2) a rigid neutral polysaccharide in aq. solns. Hematite and schizophyllan are, resp., used as model colloids. Modeling the adsorption of FA at the hematite particle surface and on the polysaccharide is based on van der Waals attractive forces and electrostatic interactions. Possible applications of the model, however, are not restricted to this system and any interaction potential or colloidal particle can be considered. The competition between FA adsorption and FA homocoagulation in soln. is studied as function of the soln. ionic strength. Results show that, under the conditions used, the amt. of adsorbed FA is largely controlled by the soln. ionic strength. At low ionic strength the amt. of adsorbed FA is limited by the electrostatic repulsion between FA at the colloid surfaces and FA monolayers are formed. By increasing the ionic strength the no. of adsorbed FA is found to increase. At a sufficiently large ionic strength, however, FA coagulation in soln. may strongly compete with FA adsorption at the hematite and polysaccharide surfaces. FA aggregates then adsorb at the colloid surfaces to form extended and porous structures. Results also suggest that FA adsorption and structure of the adsorbed layers are mainly driven by the complex interplay between electrostatic attractive and repulsive interactions.